Many codes will clear themselves if the condition that caused them is not seen for a while – typically X number of starts. The number varies from car computer to car computer. That’s probably what happened here. The error may or may not come back.
You can buy a code reader for $60-$80 that you can keep in the glove box. Test it in order to make sure that you know how to use it before you need it. Starting the car a couple of times with the gas cap loose should produce a harmless error code that you can use for testing.
The one concern I would have is whether the temperature gauge on your dash panel is driven from that temperature sensor. Driving around with a potentially faulty temperature gauge would make me nervous.
(Many Chrysler models allow you to read out simplified codes by playing with the ignition key. I wasn’t aware that GM also did that, but it is possible.)